It has been previously proposed to change the timing and/or the lift of intake and exhaust valves of internal combustion engines to the end of optimizing the performance of the engine over a wide operating range. In some cases, a plurality of intake or exhaust valves are provided in each cylinder, and only one of them is actuated while the other is kept closed in low speed operating condition of the engine. The other valve of each cylinder is also actuated in high speed operating condition of the engine.
It is generally preferred to increase the opening area and the open time period of intake and exhaust valves in high speed range of engine operation to reduce resistance to intake and exhaust flows, and to reduce the opening area and the open time period of intake and exhaust valves in low speed range of engine operation to utilize inertia effects of intake and exhaust flows to an advantage. For instance, copending U.S. patent application Ser. No. 07/388,259 filed Aug. 1, 1989 discloses a variable timing valve actuating mechanism which is provided with low speed cam lobes and high speed cam lobes in its cam shaft so that the intake and exhaust valves of the engine may be actuated according to different timing schedules depending on the operating condition of the engine.
According to this previously proposed variable timing valve actuating mechanism, a plurality of rocker arms are pivotally supported by a common rocker shaft for each of the cylinders, and by selectively coupling some of the rocker arms integrally with one another by means of slidable pins extending in parallel with the rocker shaft the lift and the timing of the valves are changed in stepwise fashion depending on the rotational speed of the engine. Each of the slidable pins is normally confined within the associated rocker arm, but it may be selectively actuated so as to fit into a guide bore of the adjoining rocker arm so as to integrally join the two adjoining rocker arms with each other.
In order to ensure a smooth and reliable operation of such a suitable pin structure, it is necessary to maintain an exact alignment of the guide bores of two adjoining rocker arms. However, due to wears of cam slipper surfaces of the rocker arms and other reasons, a misalignment between the guide bores could occur, and a smooth operation of the slidable pin may not be ensured. Further, if any failure of a control valve for supplying hydraulic pressure to actuate the slidable pin should occur, a smooth operation of the valve actuating mechanism is not possible. It is therefore preferred to monitor the operation of the valve actuating mechanism to allow a corrective measure to be taken and ensure the reliability of the engine. Based upon this recognition, the assignee of this patent application has previously proposed a variable valve actuating mechanism having means for detecting unsatisfactory operation of slidable pins in Japanese patent laid opening publication No. 63-147909.